1. Field of the Invention
The present invention relates to pilot operated hydraulic poppet valves; and more particularly to electrically operated pilot valves with a force feedback mechanism.
2. Description of the Related Art
Agricultural tractors and other types of hydraulically operated machinery commonly have components that are moved by a hydraulic actuator, such as a cylinder/piston arrangement. The piston slides within the cylinder and divides the cylinder interior into two chambers. By selectively applying hydraulic fluid under pressure to one chamber and draining hydraulic fluid from the other chamber, the piston can be forced to move in opposite directions within the cylinder. Such movement drives a rod connected between the piston and a component of the machinery.
Previously a manually operated valve assembly controlled the flow of fluid to and from the hydraulic actuator. Presently, there is a trend in agricultural equipment away from manual valves toward electrically operated valves. Electrical operation not only permits the valves to be located remotely from the operator position, but also enables computer control of the valves which allows more sophisticated functions to be provided. With electrical controls, the operator manipulates a joystick or other type of electrical input device to send signals to a microcomputer based controller, thereby indicating the desired movement of the associated components on the agricultural equipment. The controller interprets the electrical signals from the operator's input device and generates control signals which operate the hydraulic valves that control a hydraulic actuator which produces the desired motion.
A set of proportional solenoid valves, such as described in U.S. Pat. No. 5,878,647, can be used to control the flow of fluid to and from the hydraulic actuator. One solenoid valve is opened to supply pressurized fluid to the cylinder chamber on one side of the piston and another solenoid valve to exhaust fluid from the opposite cylinder chamber to a reservoir, or tank. By varying the degree to which the pair of solenoid valves are opened, the rate of flow into the associated cylinder chamber can be varied, thereby moving the piston at proportionally different speeds.
Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot valve element that opens and closes a pilot passage in a main valve poppet. Opening the pilot passage reduces pressure in a control chamber behind the main valve poppet which causes that poppet to move away from a valve seat, and allow fluid to flow through the valve.
With this type of pilot operated valve, the pilot valve element has to move the same distance as the main valve poppet is to move away from a valve seat. Therefore, if the maximum distance that the main valve poppet is to be able to open is five millimeters, the pilot valve element must be able to move five millimeters away from the main valve poppet in the closed state. If one desires to design a valve with a different maximum flow and thus movement of the main valve poppet, the pilot valve element often to be redesigned provide that movement. Therefore, it is desirable to produce valves with different flow characteristics without having to redesign most of its components.